Ok, in order to get a minimum we need destructive interference. That only occurs when the two coherent sources are exactly half a wavelength out of phase. If that sounds a bit random draw a sine wave and then draw another one starting at 180 degrees.

We know this is the first minimum so we know that the path difference between the two sources to that point is half a wavelength. If it was one wavelength we would get constructive interference and 1 and a half wavelenthes would be a second order minumum.

But since it's the first you just need to use path difference = wavelength/2

When you get that done I can help you with the uncertainties if you need it.

Now for the uncertainty you have to find out wether the distance from s1 or s2 has a larger uncertainty, and convert that into a percentage. That percentage will be the same percentage as the uncertainty in the wavelength